I'm an epidemiologist with an interest in lifestyle and environmental exposures that play a role in the development of chronic diseases. I'm particularly interested in perceptions of risk and how these square with what the science tells us. My book "Hyping Health Risks" deals with these issues. I'm in the Department of Epidemiology and Population Health at the Albert Einstein College of Medicine in New York City and am a Contributing Editor at STATS (Statistical Assessment Service) at George Mason University.

How Credible Is CDC's 43 Percent Decline In Obesity In Young Children?

We Are All Eager For Good News On The Obesity Front – But Let’s Not Be Fooled By Shaky Statistics

Yesterday’s news that the prevalence of obesity among 2-5 year-old children had decreased by a whopping 43 percent made headlines across the country. The New York Times announced on its website: “Obesity Rate for Young Children Plummets 43 Percent in a Decade.”

The much ballyhooed statistic derives from a study by researchers at National Center for Health Statistics, part of the Centers for Disease Control and Prevention (CDC), which was published yesterday in JAMA.

The researchers used data from a representative sample of the US population – the National Health and Nutrition Examination Survey (NHANES) — to examine trends in the prevalence of obesity in children and adults between 2003 and 2012.

While the media trumpeted the result, health professionals hailed the news and interpreted it as a sign that we may finally be seeing the beginnings of a reversal in the upward trend in obesity over past decades.

Actually, the JAMA paper reported the result in 2-5 year-old children as one result in an overall picture which showed no change in obesity either in children and adolescents or in adults over the 10-year period.

However, few people bothered to look at the numbers in the paper that provided the basis for the dramatic decrease – or to assess how they should be interpreted.

This one number was latched onto, taken out of context, and turned into a much more solid fact than it has any right to be. The authors themselves make a number of qualifications and cautions, but, of course, these did not make it into the headlines, or many of the news stories.

First of all, the overwhelming data in the paper for the ten-year period show no change in obesity rates. And the paper’s conclusion does not highlight – in fact, does not even mention — the result in young children. Rather, it reads in its entirety as follows:

“Overall, there have been no significant changes in obesity prevalence in youth or adults between 2003-2004 and 2011-2012. Obesity prevalence remains high and thus it is important to continue surveillance.”

The striking result that got singled out comes from Table 6.

Let’s look at the data that gave rise to the dramatic finding. The third line down shows the results for the group in question. For the 5 intervals for which data are presented, the prevalence of obesity in the 2-5 years age-group was: 13.9, 10.7, 10.1, 12.1, and 8.4 percent. Notice that the trend was decreasing over the first 3 intervals but went up in the 4th interval before going down in the fifth.

If one calculates the percent decline from 2003-2004 to 2009-2010 – rather than for the period 2003-2004 to 2011-2012 – one obtains a much less impressive 14 percent rather than 43 percent. In fact, researchers who use survey data like those presented in the JAMA paper are well aware that there are chance fluctuations in such data, and they are generally wary of seizing on short-term changes within subgroups.

The authors are aware of this and state, “The selection of the initial point can have an effect on the findings.”

In fact, because of their awareness that calculating the percent change from 2003-2004 to 2011-2012 may be shaky, in a supplementary table they use data from all years to assess the trend from 2003 to 2012. This is a more meaningful approach because it uses all of the data. In boys the trend was not significant – meaning that the change was not different from what would be expected due to chance. In girls, the trend just met the cutoff for statistical significance.

Notice also in Table 6 that the 2-5 year age-group is a subgroup within the 2-19 year age grouping, which showed no hint of a change in prevalence of obesity. Neither did either of the 2 other subgroups. This should also make us temper any weight we put on the 2-5 year age group.

In fact, the decrease is not really 43% — this number is the result of rounding the 2 percentages involved (13.9 to 14 and 8.4 to 8). If one had bothered to calculate the more appropriate value it is 39.6 percent, which could then be rounded to 40 percent – not 43 percent. But 43 percent is what was written in the press release and that’s what just about every news organization went with.

So, if the authors didn’t highlight the 43 percent in their published paper, how did it get catapulted into front-page news? By that indispensable instrument of modern science – the press release.

The CDC’s press release publicizing the results of the study zeroed in on the result in young children:

“New CDC data show encouraging development in obesity rates among 2 to 5 year olds.”

“The latest CDC obesity data, published in the February 26 issue of the Journal of the American Medical Association, show a significant decline in obesity among children aged 2 to 5 years. Obesity prevalence for this age group went from nearly 14 percent in 2003-2004 to just over 8 percent in 2011-2012 – a decline of 43 percent – based on CDC’s National Health and Nutrition Examination Survey (NHANES) data.”

What happened I think is that, not surprisingly, the researchers themselves and the publicity department at CDC decided to highlight this ray of hope in the otherwise bleak data.

This little blip in the data may or may not be the beginning of a palpable trend in obesity rates. In the CDC press release, Tom Frieden, the head of CDC, cited other indications that obesity might be beginning to decrease in young children. But, as some informed commentators pointed out regarding the JAMA paper, one would want to see more years of data before putting too much weight on the drop in obesity rates in the 2-5 year age-group.

The authors state that, in view of the seriousness of the obesity epidemic, it is important to monitor rates of overweight and obesity in the U.S. population. No one will argue with that. But we see from this study how a finding can be taken out of context and pumped up.

What the 43 percent result really has to offer is a lesson in how easy it is to over-interpret statistics, especially when they seem to point in the desired direction.

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